The purpose of this open-label, randomized, controlled, in-clinic, 5-parallel-group study was to assess biomarkers of exposure (BoE) to select harmful and potentially harmful constituents in adults who smoke (N = 144) switching to oral tobacco products (on!(& REG;) mint nicotine pouches; test products) compared to continuing smoking cigarettes (CS) and completely quitting all tobacco products (NT). Changes in 20 BoE to select harmful and potentially harmful constituents, including 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL), were evaluated. Adult smokers smoked their usual brand of cigarettes for 2 days (baseline assessments) and then were randomly assigned to ad libitum use of 2, 4, or 8 mg test products, CS, or NT for 7 days. Analysis of covariance was used to assess the Day 7 BoE levels between each group using test products, CS, and NT. The creatinine-adjusted total urinary NNAL and other 18 of 19 BoE levels (except nicotine equivalents [NEs]) were significantly lower (P < .05) on Day 7, among all test product groups compared to CS. Geometric least-square means were reduced for all biomarkers of exposure, except NEs, in test product groups by approximately 42%-96% compared to the CS group, and reductions were comparable to the NT group. The geometric least-square means for urinary NE between the test product and the CS groups, although not significantly different, the Day 7 mean change relative to the CS group were 49.9%, 65.8%, and 101% for the 2, 4, and 8 mg test product groups, respectively. The substantial reduction in harmful and potentially harmful constituent exposure suggests complete switching from cigarettes to test products may present a harm reduction opportunity for adults who smoke.